Stromal cells provide structural support for malignant cells, modulate the tumor microenvironment, and influence phenotypic behavior as well as aggressiveness of malignancies. In return, tumors provide growth factors, cytokines, and cellular signals that continually initiate new stromal reactions and recruit new cells into the microenvironment to further support tumor growth. It is not fully understood how stroma influences the neoplastic cells, but there is evidence for involvement of soluble paracrine factors, extracellular matrix formation, and direct cell-to-cell interaction, Therefore, it might be possible to manipulate tumor stroma cells and interfere with the stroma-tumor interactions for therapeutic benefit. We have demonstrated that bone marrow-derived mesenchymal stem cells (MSC) integrate into solid tumors as stromal elements and contribute to tumor development, We hypothesized that MSC would home to and selectively proliferate in the tumor microenvironment and that genemodified MSC could be used as cellular vehicles to deliver gene products into tumors. Preliminary data suggest that MSC home to and contribute to the tumor stroma formation in both subcutaneous and metastatic tumor xenografts in mice and proliferate rapidly in situ. Local production of IFN-beta by MSC resulted in pronounced anti-tumor effects. Our proposed studies aim at understanding the factors that influence MSC homing and selective proliferation in the tumor microenvironment. In addition, we will optimize the cellular delivery of therapeutic genes into the stroma of metastatic and subcutaneous breast cancer xenografts and syngenic breast cancers. Finally, we will test the hypothesis, that autologous, gene-modified MSC will home to and proliferate in metastases from breast cancer patients and produce therapeutic proteins in situ, in a gene products Phase l/ll clinical trial. SA#1: To determine the biodistribution and selective proliferation of intravenously administered MSC in the stromal microenvironment, to identify cellular mediators of this process and to determine the phenotype and fate of engrafted MSC. SA#2: To investigate the tumor targeting ability and distribution of MSC in vivo utilizing noninvasive imaging techniques. SA#3: To determine which of several viral gene delivery systems utilized for MSC transfection results in optimal tumor growth inhibition and cell killing and to determine the effects of MSC on the growth of tumors in immunocompetent and immunodeficient mice. SA#4: To test the hypothesis that autologous interferon-beta producing MSC home and proliferate in tumors from patients with refractory metastatic breast cancer and to determine the biological effects, clinical activity and safety by conducting a phase l/ll study.